Apparatus for calcining gypsum

ABSTRACT

An apparatus and process for calcining gypsum is disclosed which utilizes a flash furnace for rapidly calcining fine particulate gypsum while the material is suspended in a stream of hot gas. The method and apparatus utilize a two-stage suspension preheater for utilizing waste gases from the calcining furnace for preheating raw material to be calcined and a two-stage suspension cooler for cooling hot calcined gypsum which is discharged from the calcining furnace. Spent cooling gas is supplied to the preheater for use in preheating the fresh gypsum to be calcined. The invention also contemplates recirculating a portion of the gypsum discharged from the furnace back to the furnace for further calcination. Hot product may be discharged directly from the furnace.

This is a division of application Ser. No. 06/718,592, now U.S. Pat. No.4,569,831.

BACKGROUND OF THE INVENTION

The present invention relates to calcining gypsum while in suspension ina heated gas stream. More particularly, the invention relates to aprocess and apparatus for calcining gypsum while in suspension in a hotgas stream which permits a control over the degree of calcination to beachieved.

The raw material for gypsum is calcium, sulfate, dehydrate (CaSO₄ 2H₂ O)which can be converted by calcining to a number of phases which dependon the calcining conditions. The raw material is dehydrated atapproximately 375° F. to form the hemihydrate (2CaSO₄ H₂ O) which isbasis of most gypsum plasters and called calcined gypsum or, when usedfor making ornaments or casts, it is called Plaster of Paris. When mixedwith water it forms the hydrated sulfate that solidifies and sets firmdue to interlocking crystallization.

If calcined further to anhydrous anhydrite (CaSO₄) the material may beused as a paper filler under the name of Pearl filler. An anhydrouscalcium sulfate in powder or granular form will absorb 12% to 14% of itsweight of water and is used as a drying agent for gases and chemicals.It can be regenerated for reuse by heating the material and drying offthe water.

Prior to the present invention, most plaster manufacturing has been donein a batch calcining kettle which accomplishes the work in 21/2 hours attemperatures on the order of 280° F. to 335° F. in a 15 ton batch. Feedsize is around 11/4 inch as the largest size. The kettle product isground to produce a final product. Plaster has also been manufactured ina rotary kiln utilizing a stone feed on the order of 11/2 inch size. Theproduct goes through a hammer mill followed by a grinding mill toproduce 70% to 95% minus 100 mesh plaster product. Neither the rotarykiln system nor the batch calcining kettle of common practice has muchheat recovery and is not an energy efficient process.

Calcining gypsum in a current of hot gas is disclosed in U.S. Pat. No.3,489,994 issued Mar. 14, 1972. This patent discloses calcining thegypsum while the material is within suspension in a hot gas stream in acyclone. There may be two stages of heating with hot gas seriallysupplied first to one cyclone then to a second cyclone. The material tobe calcined is supplied first to the second cyclone and then alternatelyeither to the first cyclone or to a cooling apparatus. Water is sprayedonto the calcined product for cooling the product and controllingmoisture content. With this prior device, the moisture content in theproduct is controlled by the cooling cycle. Thermal economies are notachieved with this type of apparatus because the heat content in theproduct is not utilized in the calcining process. In addition, there isno arrangement for controlling the degree of calcination within thecalcining vessel.

Another type of apparatus for calcining gypsum in suspension isdisclosed in U.S. Pat. No. 3,956,456 issued May 11, 1976. In this devicea complex calcining furnace is utilized and there is an absence ofthermal economy in the system.

It would be desirable to provide a process and apparatus for calcininggypsum which utilizes the heat of the product in the calcining processand the spent calcining gases in a preheating stage in order to reducefuel consumption of the total process.

Prior practice has relied upon calcining temperature to vary the degreeof calcination. According to the present invention, material can berecirculated through the calcining furnace to control the amount ofcalcination. At varied temperatures and degree of calcination differentproducts of gypsum can be produced, for example 320° F. product is usedfor modeling plaster and at 650° F. gypsum plaster is produced.

SUMMARY

It is a principal object of this invention to provide a process forcalcining gypsum which provides for thermal economies and fuel savingswhen compared to prior processes.

It is a further object of this invention to provide an apparatus andprocess for calcining gypsum which utilizes spent gas from the calciningfurnace for preheating the material to be calcined and a suspensioncooler which creates gas for use in the calcining process. The processand apparatus also includes an arrangement for recirculating to thecalcining furnace a portion of the gypsum which has already passedthrough the calcining furnace to thereby provide a control for theamount of the calcination accomplished.

In general, the foregoing and other objects will be carried out byproviding a process for calcining gypsum comprising the steps of:providing a furnace means for calcining gypsum; supplying hot gas andgypsum to be calcined to the furnance means whereby the hot gas at leastpartially calcines the gypsum within the furnace means while the gypsumis suspended in the hot gas; discharging at least partially calcinedgypsum from the furnace means; separately discharging hot gas from saidfurnace means; cooling at least a portion of the at least partiallycalcined gypsum discharged from the furnace means by suspending thecalcined gypsum in cooling gas whereby the cooling gas is heated and thecalcined gypsum is cooled; preheating fresh gypsum to be calcined bysuspending the fresh gypsum in the hot gas discharged from the furnacemeans and the heated cooling gas; and selectively recirculating to thefurnace means a portion of the at least partially calcined gypsumdischarged from the furnace means for further calcining.

The apparatus of the present invention is carried out by providing anapparatus for calcining gypsum comprising a preheater for preheatingfresh gypsum to be calcined including an inlet for hot preheating gas,an inlet for fresh gypsum to be calcined and an outlet for preheatedgypsum; furnace means including an inlet for preheated gypsum flowconnected to the outlet for preheated gypsum of the preheater, means forproducing hot gas for at least partially calcining gypsum, an outlet forhot at least partially calcined gypsum and an outlet for hot gas flowconnected to the inlet for hot preheating gas of the preheater; a coolerfor cooling calcined gypsum including an inlet for hot, at leastpartially calcined gypsum flow connected to the outlet for at leastpartially calcined gypsum of the furnace means, an inlet for cooling gaswhereby heat exchange contact between the hot calcined gypsum and thecooling gas occurs within the cooler, the hot calcined gypsum is cooledand the cooling gas is heated, an outlet for cooled calcined gypsum andan outlet for heated cooling gas flow connected to the inlet for hotpreheating gas of the preheater.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in connection with a single FIGURE ofthe drawing which is a diagrammatic view of the apparatus of the presentinvention for carrying out the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus of the present invention includes a raw materialpreparation stage generally indicated at 1, a preheater generallyindicated at 2, a furnace means generally indicated at 3 and a coolergenerally indicated at 4. The apparatus will also include a highefficiency dust collection system generally indicated at 5.

The raw material preparation system 1 may include a crusherdiagrammatically indicated at 10 in which large pieces of material asmined are broken down into smaller pieces and supplied through asuitable conveying system diagrammatically illustrated at 11 to agrinding mill 12 which may be either a ball mill or a roller mill. Allof this equipment is generally known per se in the art and need not bedescribed in detail. Material is supplied through a conveying line 13 toa storage bin 14. The bin 14 may include a valve 15 for metering flowout of the bin 14.

The preheater 2 includes at least a pair of substantially identicalgas-solids separators of the cyclone type indicated at 20 and 20a. Eachcyclone includes a tangential inlet 21 for hot gas and entrained solids,a top outlet 22 for separated gas and a lower material outlet 23 forseparated preheated fresh gypsum. Each outlet may include a doubletipping valve or other air lock generally indicated at 24 forcontrolling flow of material out of the cyclone 20 while maintaining anair seal. A duct 25 connects the cyclone 20a with the inlet 21 ofcyclone 20. A duct 26 is flow connected to the inlet 21 of cyclone 20a.A conduit 27 flow connects the outlet 22 of gas-solids separator 20 withthe high efficiency dust collector means 5.

Fresh material to be calcined is supplied from the bin 14 throughconduit 16 to the duct 25 so that fresh material may be entrained in astream of hot preheating gas being supplied from gas-solids separator20a through duct 25 to gas-solid separator 20.

The conduit 16 and its juncture with conduit 25 defines an inlet 28 forfresh gypsum to be calcined. The preheater 2 also includes an outlet forpreheated gypsum and is defined by duct 29 flow connected to thedownstream side of valve 24 on cyclone 20a. According to the presentinvention, the step of preheating fresh gypsum includes sequentiallypassing the fresh gypsum through at least a pair of serially connectedgas-solids separators by alternately entraining the gypsum in a heatedgas stream and separating the gypsum from the heated gas stream.

The furnace means 3 includes a combustion chamber 30 for producing hotgas. This combustion chamber 30 is of a type generally known in the artand may be supplied with fuel and air for combustion from a suitablesource (not shown). The furnace means 3 also includes a stationaryvessel or flash calciner indicated at 32 having a lower inlet 33 for hotgas flow connected by conduit 31 to the combustion chamber 30. The inlet33 also serves as an inlet for gypsum to be calcined as will bedescribed hereinafter.

The vessel 32 includes an upper outlet 34 for hot gas and entrainedgypsum. The outlet 34 is close coupled to a gas-solids solids seperator35 of the cyclone type including a tangential inlet 34a flow connectedto outlet 34 an outlet 36 for hot spent calcining gas and an outlet 37for separated at least partially calcined gypsum. A pair of doubletipping valves 38 or other flow control means may be provide at theoutlet 37. The gas inlet conduit 26 of the preheater 2 is flow connectedto the outlet 36 to define a means for supplying hot preheating gas tothe preheater 2.

The furnace means 3 also includes a splitter 39 for dividing thecalcined product into a plurality of streams.

The cooler 4 includes at least a pair of serially connected gas-solidsseparators of the cyclone type indicated at 40 and 40a. Each separatorincludes a tangential inlet 41 for gas and entrained material, an outlet42 for separated gas and an outlet 43 for separated solids. The outlets43 may have a rotary valve 44 for controlling the flow of materialtherethrough while maintaining a seal in the cyclone. As an alternate,any other valves such as the double tipping valves of the preheatingapparatus may be utilized. A conduit 45 flow connects the outlet 42 ofseparator 40a with the gas entrained solids inlet 41 of separator 40.Hot calcined gypsum to be cooled may be supplied from distributor orsplitter 39 through a conduit 46 to conduit 45 where the material isentrained in the gas stream flowing conduit 45. The juncture of theconduit 46 which conduit 45 defines an inlet for hot at least partiallycalcined gypsum flow connected to the outlet 37 for at least partiallycalcined gypsum of the furnace means 3.

The cooler 4 also includes a conduit 47 connected at one end to a sourceof cooling gas such as a blower 50 for supplying ambient air and at itsother end to the inlet 41 of the gas solid separator 40a. A conduit 48connects the outlet 43 of cyclone 40 with the conduit 47 for supplyingmaterial to be cooled from the first cooling stage 40 for entrainment inthe cooling gas in duct 47 and supply to the second cooling stage 40a. Aconduit 49 is flow connected at one end to the outlet 42 of cyclone 40and at its other end to the conduit 26 for supplying heated cooling gasfrom the cooler 4 to the preheater 2. Thus, according to the presentinvention, the step of cooling the at least partially calcined gypsumincludes sequentially passing the at least partially calcined gypsumthrough at least a pair of serially connected gas-solids separators byalternately entraining the gypsum in a cooling gas stream and separatingthe gypsum from the cooling gas stream.

The high efficiency dust collector system 5 includes a high efficiencydust collector or baghouse 55 with an outlet through fan 59 connected tostack 60. An outlet 56 which supplies collected dust to a conveyor belt57 and conduit 58 for supply to conduit 26 and thus is returned to thesystem.

In the calcining operation, fresh material to be calcined is suppliedfrom hopper 14 through conduit 16 to inlet 28 of the preheater 2. In theduct 25 the raw material is entrained in a flow of hot gas into thecyclone 20. Gas is discharged through conduit 27 to the high efficiencydust collector. While the material is entrained in the hot gas, it ispreheated by the hot gas and then separated from the hot gas in cyclone20. The partially preheated material is discharged through outlet 23 toduct 26 where it is entrained in hot gas and supplied to second stagepreheater 20a. While in the duct 26 and in cyclone 20a the fresh gypsumto be calcined is further preheated. In the cyclone 20a hot gas isdischarged through outlet 22 to duct 25 and preheated material isdischarged through 23 to duct 29 and out of the preheater stage 2.Preheated material supplied to the furnace means 3 through conduit 29may have a temperature of approximately 250° F.-450° F.

The duct 29 is flow connected to the conduit 31 downstream of thecombustion chamber 30 and upstream of the inlet 33 of calcining vessel32. The gypsum is exposed to the hot gases from the combustion chamber30 and entrained and conveyed in these hot gases to the stationaryvessel 32. In the vessel 32 the gypsum is at least partially calcined.The vessel 32 is preferably maintained at a temperature in the range of300° F. to 650° F.

The hot gas from the vessel 32 is discharged along with entrained atleast partially calcined gypsum through conduit 34 to close coupledcyclone 35 wherein the hot gas is separated from the particulatematerial and supplied to the preheater 2 through conduit 26. The atleast partially calcined material is discharged through outlet 37 todistributor 39. From distributor 39 material may flow either throughconduit 46 to the cooler 4 or be recirculated through conduit 65 to theduct 31 for recirculation through the furnace means 3. Also, the productmay be discharged as hot product through conduit 66.

The cooler include supplying cooling gas (ambient air) from a sourcesuch as blower 50 through conduit 47 to cyclone 40a. The cooling gasthen flows through duct 45 where it entrains material being suppliedfrom conduit 46 and the furnace means 3 and conveys this material tocyclone 40. While in the conduit 45 and cyclone 40 the material iscooled and the cooling gas is heated by the hot material. The thusheated cooling gas is supplied through conduit 49 to the conduit 26 andpreheater 2 to serve as a source of hot gas for preheating the freshgypsum to be calcined. Material discharged from the first stage coolingcyclone 40 is supplied by duct 48 for entrainment in the cooling gassupplied through conduit 47. Cooled product is discharged through outlet70. Cooled product will have a temperature in the range of 85° F.-150°F. Hot gas supplied from the cooler to the preheater through conduit 49may have a temperature of approximately 200° F.-450° F.

According to the process of the present invention, there is preferably arecirculating load of up to 500%. This means that the amount of calcinedmaterial which is recirculated through duct 65 to conduit 31 for passagethrough the furnace means 3 a subsequent time may be as much as fivetimes the amount of fresh material being supplied through conduit 16 tothe inlet of preheater 2. This recirculation of material assures ahigher degree of calcination of the gypsum. The percentage recirculationwill depend on the degree of calcination desired. By having a higherrecirculating load, the material passes through the furnace 3 a greaternumber of times effectively increasing the residence time of material inthe furnace.

With the present invention, the residence time of the gypsum in thefurnace 2 is on the order of 2.5 seconds. With such a residence time,gypsum raw material having a 21.5% loss on ignition (LOI) at 1652° F.may be reduced to a 12.28% on ignition LOI at 1652° F. when beingsubjected to temperatures which will achieve a product discharge atoutlet 37 of approximately 250° F. to approximately 400° F. If thecirculating load is increased so that material passes through thefurnace a second time so that the total residence time of the materialwithin the furnace means 3 is increased to 5 seconds, the product losson ignition may be on the order of 5.6%. Thus, it can be seen that therecirculation system of the present invention permits a higher degree ofproduct calcination. As a result, by controlling recirculation there isa control over the product being produced.

From the foregoing it should be apparent that the objects of thisinvention have been carried out. An apparatus and process have beenprovided which utilizes the thermal economy achieved by the use of asuspension preheater followed by a suspension calcining system followedby a suspension cooler with the preheated air from the cooling apparatusbeing utilized for preheating the raw material and the spent hotcalcining gases also being used for preheated the raw material. Thisutilization of hot gases can result in a substantial saving in fuelconsumption and thus operating costs. Also with the present invention,the degree of calcination of the product can be controlled by therecirculation system employed. With the present invention residence timein the furnace means is short so that the fine gypsum is exposed to heatfor a short period of time. Residence time is increased by increasingrecirculation. This can be done in stages by, for example, increasingrecirculating load incrementability such as from 100% to 200% until thedesired degree of calcination is achieved. If, during operation theproduct becomes over burned, residence time can easily be decreased. Inaddition, the process and apparatus of the present invention can be usedto produce several different gypsum products such as the hemihydrate(CaSO₄ ·2H₂ O)or anhydrous anhydrite (CaSO₄).

The foregoing is intended to be a description of a preferred embodiment.It is intended that the invention be limited solely by that which iswithin the scope of the appended claims.

I claim:
 1. Apparatus for calcining gypsum comprising:a preheater forpreheating fresh gypsum to be calcined including an inlet for hotpreheating gas, an inlet for fresh gypsum to be calcined and an outletfor preheated gypsum; furnace means including an inlet for preheatedgypsum flow connected to the outlet for preheated gypsum of thepreheater, means for producing hot gas for at least partially calciningthe gypsum, an outlet for hot at least partially calcined gypsum, and anoutlet for hot gas flow connected to the inlet for hot preheating gas ofthe preheater; a cooler for cooling calcined gypsum including an inletfor hot at least partially calcined gypsum flow connected to the outletfor at least partially calcined gypsum of the furnance means, an inletfor cooling gas whereby heat exchange contact between the hot calcinedgypsum and the cooling gas occurs within the cooler, the hot calcinedgypsum is cooled and the cooling gas is heated, an outlet for cooledcalcined gypsum and an outlet for heated cooling gas flow connected tothe inlet for hot preheating gas of the preheater.
 2. Apparatus forcalcining gypsum according to claim 1 wherein said furnace meansincludes a stationary vessel, a combustion chamber defining said meansfor producing hot gas, said vessel having a lower inlet for hot gas flowconnected to said combustion chamber, an upper outlet for hot gas andentrained at least partially calcined gypsum and a gas-solids separatorclose coupled to the outlet of the stationary vessel for separating theat least partially calcined gypsum from the hot gas and having an outletfor hot gas defining the outlet for hot gas of the furnace means that isflow connected to the inlet for preheating gas of the preheater, and anoutlet for separated at least partially calcined gypsum that defines theoutlet for hot at least partially calcined gypsum of the furnace means.3. Apparatus for calcining gypsum according to claim 2 furthercomprising means for selectively recirculating at least a portion of theat least partially calcined gypsum from the outlet of the gas-solidsseparator of the furnace means to the inlet for preheated gypsum of thefurnace means for further calcining the gypsum.
 4. Apparatus forcalcining gypsum according to claim 3 wherein said preheater includes atleast a pair of serially connected gas-solids separators for alternatelyentraining in the preheating gas fresh gypsum to be calcined andseparating the fresh gypsum from the preheating gas.
 5. Apparatus forcalcining gypsum according to claim 4 wherein said cooler includes atleast a pair of serially connected gas-solids separators for alternatelyentraining in said cooling gas the hot at least partially calcinedgypsum and separating cooled gypsum from the cooling gas.
 6. Apparatusfor calcining gypsum according to claim 3 wherein said means forrecirculating at least a portion of the at least partially calcinedgypsum includes a splitter for selectively discharging at least aportion of the at least partially calcined gypsum from the apparatus. 7.Apparatus for calcining gypsum according to claim 3 wherein said meansfor recirculating at least a portion of the at least partially calcinedgypsum includes conduit means flow connecting the outlet of the gassolids separator to the outlet of the combustion chamber.
 8. Apparatusfor calcining solid particulate material comprising:a preheater forpreheating fresh particulate material to be calcined including an inletfor hot preheating gas, an inlet for fresh particulate material to becalcined and an outlet for preheated material; furnace means includingan inlet for preheated particulate material flow connected to the outletfor preheated material of the preheater, means for producing hot gas forat least partially calcining the particulate material, an outlet for hotat least partially calcined particulate material, and an outlet for hotgas flow connected to the inlet for hot preheating gas of the preheater;a cooler for cooling calcined particulate material including an inletfor hot at least partially calcined particulate material flow connectedto the outlet for at least partially calcined particulate material ofthe furnace means, an inlet for cooling gas whereby heat exchangecontact between the hot calcined particulate material and the coolinggas occurs within the cooler, the hot calcined particulate material iscooled and the cooling gas is heated, an outlet for cooled calcinedparticulate material and an outlet for heated cooling gas flow connectedto the inlet for hot preheating gas of the preheater.
 9. Apparatus forcalcining particulate material according to claim 8 wherein said furnacemeans includes a stationary vessel, a combustion chamber defining saidmeans for producing hot gas, said vessel having a lower inlet for hotgas flow connected to said combustion chamber, an upper outlet for hotgas and entrained at least partially calcined particulate material and agas-solids separator close coupled to the outlet of the stationaryvessel for separating the at least partially calcined solid particulatematerial from the hot gas and having an outlet for hot gas defining theoutlet for hot gas of the furnace means that is flow connected to theinlet for preheating gas of the preheater, and an outlet for separatedat least partially calcined particulate material that defines the outletfor hot at least partially particulate material of the furnace means.